(1) Field of the Invention
The present invention relates to a cutting-off control apparatus and method for a printing machine which are suitable for cutting off at a predetermined position a web having images printed thereon.
(2) Description of the Background Art
In conventional rotary web printing machines, etc., there is provided a web cutting-off control apparatus which controls a cutting-off position so that, when a web after being printed is cut off in the web width direction, the cutting-off position does not shift in the web flowing direction (e.g., see Japanese Patent Laid-Open Publication No. Hei 5-330022).
As shown in FIG. 10, a web 1 is conveyed from a paper feed section 51 through a series of guide rollers 52 and to a printing unit 53, in which images are transferred to the web 1. Thereafter, the web 1 is dried by a dryer section 54, passes through a web pass section 55, and is conveyed to a folding machine 56, in which the web 1 is cut off in the web width direction by a sawing cylinder 60. At this time, a cutting-off control apparatus controls the cutting-off position of the web 1 so that the web 1 is cut off at a predetermined position.
That is, in the control of the cutting-off position by the cutting-off control apparatus, a cut register mark (hereinafter referred to as a cut mark or simply a mark) printed on the web 1 is detected by a mark detector 57, and based on a detection signal from the mark detector 57 and a reference pulse from an encoder 61 rotating in synchronization with the sawing cylinder 60 of the folding machine 56, a controller 62 shown in FIG. 10 drives a motor 58 so that the timing at which the web 1 is cut off by the sawing cylinder 60 and the timing at which the mark is detected by the mark detector 57 coincide with each other or are within a predetermined difference. The rotation of the motor 58 causes a compensator roller 59 to move in the up-and-down direction. Since the up-and-down movement of the compensator roller 59 can finely adjust the travel path length of the web 1 to finely adjust the phase of the web 1 relative to the rotation phase of the sawing cylinder 60, the cutting-off position of the web 1 can be kept constant. However, in this example, while the compensator roller 59 is controlled in the up-and-down direction, it may be controlled in a different direction (e.g., a right-and-left direction), depending upon the arrangement of the compensator roller 59. That is to say, the compensator roller 59 may be moved in any direction, so long as the movement thereof can finely adjust the travel path length of the web 1.
The above-described cut mark is normally formed into a slender rectangle extending in the web width direction, and it is standard to print the cut mark outside an image area, but in recent years, there has been developed a cutting-off control technique which does not print an image and a separate mark together but considers a specified portion of an image as a cut mark. In the following description, such a specified portion of an image considered as a cut mark refers to as a mark equivalent or simply a mark.
On the other hand, the mark detector 57 is a sensor that responds to light reflected from the web 1, and it is a matter of course that the detection area (visual field) thereof is limited. The mark detector 57 cannot detect a mark if it does not pass through the visual field thereof. As shown in FIG. 11A, in the case where the mark 2 is at a predetermined constant position such as a position which is within a printable area between images 3 printed on the web 1 and near one end in the width direction of the web 1, if the mark detector 57 is installed according to the mark position, the mark 2 can pass through the visual field of the mark detector 57. However, if the mark position is shifted widthwise from the constant position, the mark 2 will pass through a position shifted from the visual field of the mark detector 57 and therefore it will not be detected.
Hence, the inventors have proposed a technique for controlling the width direction of the mark detector 57 according to the position in the width direction of a mark (see Japanese Patent Laid-Open Publication No. 2004-82279).
In this technique, the resolution of image data for plate making (or image data obtained by processing the image data for plate making) is converted to the resolution of the mark detector. And based on the converted image data, the position of a mark on a web is calculated, and based on the calculated mark position, the mark detector is moved to the mark position in the web width direction before the start of printing. This makes it possible to detect the mark on the web at the start of printing and to keep the web cutting-off position constant, whereby waste paper can be considerably reduced.
The above-described mark detector detects the mark on the web by a change in light quantity (luminous intensity) reflected from the web which travels. That is, a light quantity reflected from the web is strong for white paper, but becomes very weak at a place where a large quantity of ink is transferred, such as solid printing (halftone area ratio=100%), like a cut mark. Therefore, a rapid change in reflected light quantity (strong to weak, or weak to strong) makes it possible to detect the cut mark.
However, since the web surface (printed surface) which passes through the detection area of the mark detector does not always have only cut marks, there is a possibility that a part not being a cut mark will be incorrectly recognized as a mark.
That is, as shown in FIG. 11A, in the case where the cut mark 2 is present in the margin shifted in the web width direction from the printing area of the image 3 on the web 1, a part changing sharply in reflected light quantity can be recognized as the cut mark 2. However, as shown in FIG. 11B, in the case where the mark 2 is present in the margin between the image printing areas, the image 3 also pass through the detection area of the mark detector 57. Therefore, if the image 3 contains a part changing sharply in reflected light quantity, there is a possibility that this part will be incorrectly recognized as the mark 2. In addition, as shown in FIG. 11C, in the case where an edge portion 5 in the image 3 is handled as a mark equivalent, if the remaining part in the image 3 passing through the detection area of the mark detector 57 contains a portion changing sharply in reflected light quantity, there will be possibility that the portion will be incorrectly recognized as the mark 2.
Thus, if the cut mark, including the mark equivalent, is incorrectly recognized, the web will cut off at an inappropriate position and therefore the operator will need to make adjustments. In addition, all of the sheets cut off during this period will become waste paper.
As shown in FIGS. 11A to 11C, although the mark 2, or the edge portion 5 of the image 3 as the mark equivalent, is a specified part in the traveling direction of the web 1, the above-described incorrect mark recognition is caused by the mark detecting system that detects the mark or mark equivalent over all of the area in the traveling direction of the web 1. In addition, such a system causes a great load to memory and arithmetic systems associated with mark recognition.